[PATCH v2] x86: Perform mem_sharing teardown before paging teardown

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

An assert failure has been observed in p2m_teardown when performing vm
forking and then destroying the forked VM (p2m-basic.c:173). The assert
checks whether the domain's shared pages counter is 0. According to the
patch that originally added the assert (7bedbbb5c31) the p2m_teardown
should only happen after mem_sharing already relinquished all shared pages.

In this patch we flip the order in which relinquish ops are called to avoid
tripping the assert.

Signed-off-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
Fixes: e7aa55c0aab3 ("x86/p2m: free the paging memory pool preemptively")
---
v2: comsetic fixes
---
 xen/arch/x86/domain.c | 56 ++++++++++++++++++++++---------------------
 1 file changed, 29 insertions(+), 27 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index db3ebf062d..a42f03e8e5 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -2289,9 +2289,9 @@ int domain_relinquish_resources(struct domain *d)
 
         enum {
             PROG_iommu_pagetables = 1,
+            PROG_shared,
             PROG_paging,
             PROG_vcpu_pagetables,
-            PROG_shared,
             PROG_xen,
             PROG_l4,
             PROG_l3,
@@ -2310,6 +2310,34 @@ int domain_relinquish_resources(struct domain *d)
         if ( ret )
             return ret;
 
+#ifdef CONFIG_MEM_SHARING
+    PROGRESS(shared):
+
+        if ( is_hvm_domain(d) )
+        {
+            /*
+             * If the domain has shared pages, relinquish them allowing
+             * for preemption.
+             */
+            ret = relinquish_shared_pages(d);
+            if ( ret )
+                return ret;
+
+            /*
+             * If the domain is forked, decrement the parent's pause count
+             * and release the domain.
+             */
+            if ( mem_sharing_is_fork(d) )
+            {
+                struct domain *parent = d->parent;
+
+                d->parent = NULL;
+                domain_unpause(parent);
+                put_domain(parent);
+            }
+        }
+#endif
+
     PROGRESS(paging):
 
         /* Tear down paging-assistance stuff. */
@@ -2350,32 +2378,6 @@ int domain_relinquish_resources(struct domain *d)
             d->arch.auto_unmask = 0;
         }
 
-#ifdef CONFIG_MEM_SHARING
-    PROGRESS(shared):
-
-        if ( is_hvm_domain(d) )
-        {
-            /* If the domain has shared pages, relinquish them allowing
-             * for preemption. */
-            ret = relinquish_shared_pages(d);
-            if ( ret )
-                return ret;
-
-            /*
-             * If the domain is forked, decrement the parent's pause count
-             * and release the domain.
-             */
-            if ( mem_sharing_is_fork(d) )
-            {
-                struct domain *parent = d->parent;
-
-                d->parent = NULL;
-                domain_unpause(parent);
-                put_domain(parent);
-            }
-        }
-#endif
-
         spin_lock(&d->page_alloc_lock);
         page_list_splice(&d->arch.relmem_list, &d->page_list);
         INIT_PAGE_LIST_HEAD(&d->arch.relmem_list);
-- 
2.34.1